Does the preinstalled thermal pad have a protective covering? Many of them have a plastic film that has to be removed before installing in order to make good thermal contact with the GPU. Check for that.
All the video cards I've put waterblocks (about 12 of them) on work fine without a fan connected. The remaining possibility is a short. Are there projections from the block to cool VRM MOSFETS? Are those supposed to use thermal pads? Most do and those pads provide electrical insulation where the MOSFETS can have metal tabs. Check for those, too.
Any sign of heat? Scorching or other discoloration that might come from a short? Check for a short near the PSU cable sockets. A dead short on 12v would be safely caught by the PSU overcurrent protection and prevent booting without permanent damage.